Mercedes-Benz ML320. Manual — part 647

Fig. 3: Connecting Test Cable & Socket Box To A/C Push-Button Control Unit
Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

2001 Mercedes-Benz ML320

2000-01 MANUAL A/C-HEATER SYSTEMS ML 320, ML 430 & ML 55

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FUNCTIONAL TEST PREPARATION

1. Check condition of fuses F16, F41, F43 and F44. See WIRING DIAGRAMS . Repair and/or replace

fuses and necessary.

2. Turn ignition on. Locate in-vehicle temperature sensor aspirator blower vent grille above anti-theft

indicator light. Place a half-inch square of tissue paper over aspirator blower vent grille. If paper stays on
vent grille, go to next step. If paper does not stay on vent grille, correct insufficient aspirator blower
ventilation.

3. Put shift lever in "P" and engage parking brake. Run engine at idle until it reaches normal operating

temperature, about 176°F (80°C). Ensure ambient (outside) temperature is greater than 58°F (15°C).

4. Manually open center and side air outlets. Ensure recirculation button is not depressed. Set blower speed

knob to position No. 1.

FUNCTION TEST

Defrost Mode

To check operation in defrost mode, set temperature selector knob to White range (vertical). Set mode control
knob to 12-o'clock position (vertical). Ensure A/C button indicator is illuminated. Air will vent from defrost and
center outlets and A/C compressor will engage. If operation is not as specified, perform the following in order:



Check voltage supply circuit Z50/2 between No. 16 fuse (15-amp) and connected components. See
VOLTAGE SUPPLY CIRCUIT Z50/2 under ELECTRICAL TESTS.



Check ground circuit. See GROUND CIRCUIT Z50/4 under ELECTRICAL TESTS.



Check in-vehicle temperature sensor signal voltage. See IN-VEHICLE TEMPERATURE SENSOR
under ELECTRICAL TESTS.



Check in-vehicle temperature sensor signal resistance. See IN-VEHICLE TEMPERATURE SENSOR
under ELECTRICAL TESTS.



Check icing protection temperature sensor signal voltage. See ICING PROTECTION
TEMPERATURE SENSOR under ELECTRICAL TESTS.



Check icing protection temperature sensor resistance. See ICING PROTECTION TEMPERATURE
SENSOR under ELECTRICAL TESTS.



Check activation voltage during temperature reduction. See ACTIVATION VOLTAGE
TEMPERATURE under ELECTRICAL TESTS.



Check activation voltage during temperature increase. See ACTIVATION VOLTAGE
TEMPERATURE under ELECTRICAL TESTS.



Check blend air flap actuator motor operation voltage. See BLEND AIR FLAP ACTUATOR MOTOR
under ELECTRICAL TESTS.



Check blend air flap actuator motor resistance. See BLEND AIR FLAP ACTUATOR MOTOR under
ELECTRICAL TESTS.



Check A/C button activation. See A/C BUTTON under ELECTRICAL TESTS.



Check A/C pressure switch. See A/C REFRIGERANT PRESSURE SWITCH under ELECTRICAL

NOTE:

Function test steps must be performed in order given.

2001 Mercedes-Benz ML320

2000-01 MANUAL A/C-HEATER SYSTEMS ML 320, ML 430 & ML 55

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TESTS.

Normal Ventilation In Regulating Mode

To check normal ventilation in regulating mode, set temperature selector knob to White range (vertical). Set
mode control knob to 4-o'clock position. Ensure A/C button indicator is illuminated. Air will vent from upper
and lower outlets and A/C compressor will engage. Ambient air will flow from center outlet and coolant
circulation pump will operate. If operation is not as specified, perform the following in order:



Check A/C button activation. See A/C BUTTON under ELECTRICAL TESTS.



Check A/C pressure switch. See A/C REFRIGERANT PRESSURE SWITCH under ELECTRICAL
TESTS.

Economy Setting Not In Heating Mode

To check economy setting without heating mode activated, set temperature selector knob to Blue range. Set
mode control knob to the 9-o'clock position. Ensure A/C button indicator is not illuminated. Ambient air will
flow from center outlet and A/C compressor will NOT be engaged. If operation is not as specified, perform the
following:



Check A/C button activation. See A/C BUTTON under ELECTRICAL TESTS.

Economy Setting In Heating Mode

To check economy setting with heating mode activated, set temperature selector knob to Red range. Set mode
control knob to the 4-o'clock position. Ensure A/C button indicator is not illuminated. Heated air will flow from
lower, upper and center outlets and A/C compressor will NOT be engaged. If operation is not as specified,
perform the following:



Check A/C button activation. See A/C BUTTON under ELECTRICAL TESTS.

Recirculation Mode

To check recirculation mode, press fresh/recirculation button. Ensure fresh/recirculation indicator is
illuminated. Set blower speed knob to position No. 4. Blower fan noise will increase noticeably. If operation is
not as specified, check wiring between recirculated air flap element, heater-A/C switch and No. 16 fuse (15-
amp).

ELECTRICAL TESTS

Voltage Supply Circuit Z50/2

1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST

EQUIPMENT . Turn ignition on, with engine off. Connect voltmeter positive lead to socket box terminal
No. 7 and negative lead to terminal No. 9.

2. Voltage should be 11-14 volts. If voltage is not as specified, check voltage supply circuit Z50/2 between

No. 16 fuse (15-amp) and connected components. See WIRING DIAGRAMS . Repair as necessary.

2001 Mercedes-Benz ML320

2000-01 MANUAL A/C-HEATER SYSTEMS ML 320, ML 430 & ML 55

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Ground Circuit Z50/4

1. Connect socket box to A/C push button control unit wiring harness connector. See CONNECTING

TEST EQUIPMENT . Turn ignition off. Ensure A/C push button control unit is not connected to socket
box.

2. Connect ohmmeter positive lead to socket box terminal No. 9 and negative lead to body ground.

Resistance should be zero. If resistance is not as specified, check control unit ground circuit Z50/4. See
WIRING DIAGRAMS . Repair as necessary.

In-Vehicle Temperature Sensor

1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST

EQUIPMENT . Connect voltmeter positive lead to socket box terminal No. 12 and negative lead to
terminal No. 9.

2. Turn ignition on. Turn temperature control knob to Red range detent. Voltage should be 1.9 volts at 68°F

(20°C). If voltage is as specified, go to next step. If voltage is not as specified, check in-vehicle
temperature sensor circuits. See WIRING DIAGRAMS . Repair as necessary. If circuits are okay,
replace in-vehicle temperature sensor.

3. Turn ignition off. Disconnect test cable from A/C push button control unit. Connect ohmmeter positive

lead to socket box terminal No. 2 and negative lead to terminal No. 12. Measure temperature sensor
resistance at specified temperatures. See IN-VEHICLE TEMPERATURE SENSOR RESISTANCE
table.

4. If resistance is not as specified, check wiring harness and connectors between A/C push button control

unit, in-vehicle temperature sensor, blend air flap actuator position sensor and temperature regulator
switch. Repair as necessary.

IN-VEHICLE TEMPERATURE SENSOR RESISTANCE

Icing Protection Temperature Sensor

1. Connect socket box to A/C push button control unit and wiring harness. See CONNECTING TEST

EQUIPMENT . Connect voltmeter positive lead to socket box terminal No. 11 and negative lead to
terminal No. 9.

2. Turn ignition on. Voltage should be 2.0-2.4 volts at 32°F (0°C) and 1.4-1.8 volts at 59°F (15°C). If

voltage is as specified, go to next step. If voltage is not as specified, check icing protection temperature
sensor circuits. See WIRING DIAGRAMS . Repair as necessary. If circuits are okay, replace icing
protection temperature sensor.

3. Turn ignition off. Disconnect test cable from A/C push button control unit. Connect ohmmeter positive

lead to socket box terminal No. 3 and negative lead to terminal No. 11. Measure temperature sensor
resistance at specified temperatures. See ICING PROTECTION TEMPERATURE SENSOR
RESISTANCE table.

Sensor Temperature - °F (°C)

Ohms

68 (20)

2100

77 (25)

1700

104 (40)

900

2001 Mercedes-Benz ML320

2000-01 MANUAL A/C-HEATER SYSTEMS ML 320, ML 430 & ML 55

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